Definition: Nodular fasciitis is a benign proliferation of fibroblasts in the subcutaneous tissues, commonly associated with the deep fascia. Nodular fasciitis is a benign mesenchymal tumor. Often presenting as a rapidly enlarging soft tissue mass, clinically, it can easily be mistaken as a sarcoma or other malignancy during clinical evaluation.

Nodular fasciitis (NF) is a relatively common mass-forming and self-limited subcutaneous pseudosarcomatous myofibroblastic proliferation of unknown pathogenesis. Due to its rapid growth and high mitotic activity, NF is often misdiagnosed as a sarcoma.

Reports of nodular fasciitis among adults are common; however, this condition is relatively rare in the pediatric population.

Nodular (pseudosarcomatous) fasciitis is a presumably reactive vascular and fibroproliferative response to injury.

The lesion is benign but has a rapid rate of growth and a histopathologic appearance which can be quite alarming.

Although relatively common, it was not recognized as a separate histopathologic entity until 1955.

Clinical synopsis

Approximately 17% of all cases occur in the head and neck region, usually the neck and face.

In the mouth, nodular fasciitis is usually a discrete submucosal nodule which is slightly tender and is not freely movable beneath the mucosa.

It seldom achieves more than 2 cm. in size and is usually more superficially located than fibromatosis of the oral region.

While occurring at all ages, this entity is most often diagnosed in persons 30-40 years of age, with no gender predilection.

Macroscopy

small and solitary subcutaneous nodule solid lesion, 5 or 20 mm in diameter rapid growth (less than a month in 50%) localization:

any superficial soft tissue of the body, including the breast, mucosal surfaces, bladder, and parotid gland.

Microscopy

Nodular fasciitis presents as haphazardly arranged bundles of fibroblasts in a myxoid or mucoid background. An important diagnostic feature is a fine capillary network arranged in a radial pattern around a larger central vessel or vessels. The fibroblasts are typically large and plump, similar to those of granulation tissue. Pleomorphic fibroblasts may be present, and mitoses are common but not plentiful or abnormal. A variable amount of collagen and acid mucopolysaccharide are seen in the intercellular matrix, although the latter may not be readily visible without special staining with alcian blue or colloidal iron. Scattered chronic inflammatory cells are typically present in small to moderate numbers, and long-standing lesions may demonstrate foamy histiocytes and osteoclast-like multinucleated giant cells with 2-6 nuclei. When striated muscle is involved (intramuscular fasciitis), it is completely replaced by the fibrovascular proliferation, unlike proliferative myositis, which infiltrates between muscle fibers. The spindle-shaped fibroblasts in this lesion tend to be arranged in long fascicles which are slightly curved, whorled or S-shaped. They seem especially prone to extension along the fibrous septa of submucosal fatty tissues. Small slit-like spaces often separate the fibroblasts, and extravasation of erythrocytes is commonly seen, although it is seldom extensive. Occasional microcysts are seen in older lesions, perhaps coalesced into larger cystic spaces. The lesion is not often encapsulated but is usually well demarcated from surrounding tissues.

Morphological synopsis

benign proliferation of fibroblasts and myofibroblasts in the subcutaneous tissues haphazardly arranged bundles of fibroblasts (medium cellularity) myxoid or mucoid background fine capillary network arranged in a radial pattern around a larger central vessel or vessels fibroblasts are typically large and plump, similar to those of granulation tissue pleomorphic fibroblasts common mitoses spindle-shaped fibroblasts arranged in long fascicles slightly curved, whorled or S-shaped extension of the spindle-shaped fibroblasts along the fibrous septa of submucosal fatty tissues. variable amount of collagen and acid mucopolysaccharide in intercellular matrix scattered chronic inflammatory cells foamy histiocytes and osteoclast-like multinucleated giant cells in long-standing lesions small slit-like spaces often separate the fibroblasts extravasation of erythrocytes occasional microcysts not often encapsulated but usually well demarcated from surrounding tissues

Ultrastructure

Ultrastructural studies have confirmed the presence of myofibroblasts in nodular fasciitis, with a basic fibroblast appearance but with peripherally located bundles of myofilaments with dense patches similar to those of smooth muscle cells.

myofibroblasts

basic fibroblastic appearance with peripherally located bundles of myofilaments with dense patches similar to those of smooth muscle cells

Immunochemistry

Lesional cells are immunoreactive to vimentin, smooth muscle actin, and muscle-specific actin, but not for desmin.

A genomic rearrangements of the USP6 locus were found in 92% (44 of 48) of NF.

Rapid amplification of 5’-cDNA ends identified MYH9 as the translocation partner. RT-PCR and direct sequencing revealed the fusion of the MYH9 promoter region to the entire coding region of USP6.

Considering the self-limited nature of the lesion, NF may represent a model of ’transient neoplasia’, as it is the first example of a self-limited human disease characterized by a recurrent somatic gene fusion event.

Differential diagnosis

The pathologist may recognize its high cellularity, high mitotic index, and infiltrative borders, which, as a result, may lead to erroneous diagnosis as a malignancy.

The differential diagnosis of this lesion includes fibrosarcoma, fibrous histiocytoma and liposarcoma, few of which demonstrate the vascular component of nodular fasciitis.

The scattered inflammatory cells also help to differentiate the lesion.

Occasional lesions will demonstrate very small foci of metaplastic bone or cartilage (ossifying fasciitis, fasciitis ossificans, parosteal fasciitis), tempting the pathologist to diagnose the case as osteosarcoma.

Its clinical and histologic characteristics are similar to malignancies such as sarcoma.

Despite its often aggressive microscopic appearance, nodular fasciitis is a self-limiting lesion which is readily treated by simple local excision.

Deeper lesions tend to be somewhat larger and less well demarcated, hence, require a wider local excision.

Recurrence rates vary from 1-6% with this treatment and some lesions have been reported to regress and disappear without treatment.

The major prognostic factor here is an accurate diagnosis and recurrences should, therefore, be evaluated very carefully.

Earlier studies have shown that as many as one-fourth of all cases were erroneously interpreted as malignant and, conversely, numerous cases of well-differentiated fibrosarcoma have been misdiagnosed as nodular fasciitis.

These lesions rarely recur, do not develop metastases, and are readily cured by local excision. Despite its often aggressive microscopic appearance, nodular fasciitis is a self-limiting lesion which is readily treated by simple local excision. Deeper lesions tend to be somewhat larger and less well demarcated, hence, require a wider local excision. Recurrence rates vary from 1-6% with this treatment and some lesions have been reported to regress and disappear without treatment.